The 1957-D Franklin Half Dollar is one of the most common issues in the series. It has a mintage of almost 20 million coins struck. While most other dates in the series have mintages well under 15 million.
This issue is common even up to about MS65 condition. In MS66 it becomes much more difficult but still out there and can be easily purchased for about a couple hundred dollars. In MS67 or higher, it becomes very scarce and it is very difficult to obtain. Any example in this condition or higher will be very expensive since the demand is much greater than the supply.